Generally, this invention relates to the improved production of coalbed gas from substantially solid subterranean formations including coalbeds. Specifically, this invention relates to the use of a stimulation gas to manipulate the physical and chemical properties of such subterranean formations and to increasing the quantity, quality and rate of production of coalbed gases associated with such subterranean formations.
A significant quantity of coalbed gas is physically bound (or sorbed) within coalbeds. This coalbed gas, which was formed during the conversion of vegetable material into coal, consists primarily of methane. Because it is primarily methane, coal gas is commonly termed coalbed methane. Typically, more than 95% of the coalbed methane is physically bound (adsorbed) onto the surface of the coalbed matrix.
Coal may be characterized as having a dual porosity character, which consists of micropores and macropores. The micropore system is contained within the coal matrix. The micropores are thought to be impervious to water; however, the vast majority of coalbed methane contained by the coalbed is adsorbed onto the walls associated with the micropores. The macropores represent the cleats within the coal seam. Face and butt cleats are interspersed throughout the coal matrix and form a fracture system within the coalbed. The face cleats are continuous and account for the majority of the coalbed's permeability. Butt cleats are generally orthogonal to the face cleats but are not continuous within the coal. On production, the coalbed matrix feeds the cleat system and the desorbed coalbed gas is subsequently removed from the coalbed at production wells.
Several important problems limit the economic viability of coalbed methane production. The first is the handling of produced water from water-saturated coalbeds. The handling of produced water can be a significant expense in coalbed methane recovery. In a typical water-saturated reservoir, water must first be depleted to some extent from the cleat system before significant coalbed methane production commences. Water handling involves both pumping and disposal costs. If the coalbed is significantly permeable and fed by an active aquifer, it may be impossible to dewater the coal and induce gas production. Production of significant quantities of water from an active aquifer may be legally restricted and may result in lawsuits from others who rely on the affected water supply. Disposal of the produced water can present several problems. The water may be discharged to the surface and allowed to evaporate. If sufficiently clean, the water may be used for agricultural purposes. Finally, the water may be reinjected into the coal. All of these disposal methods require environmental permitting and are subject to legal restrictions. Many conventional coalbed gas production systems only displace water in the vicinity of the production well which results in a short coalbed gas production period which lasts only hours or a few days. One example is disclosed in U.S. Pat. No. 4,544,037. Gas production stops when the water returns to the coalbed surrounding the production well.
The second problem which limits the economic viability of coalbed gas production is maintaining the appropriate removal rate of coalbed gas as it is desorbed from the coalbed. As the pressure in the immediate vicinity of the producer decreases, a quantity of gas desorbs from the coal and begins to fill the cleat system. If the water is excluded from the coalbed surrounding coalbed gas production well, and as gas desorption continues, the gas phase becomes mobile and begins to flow to the low-pressured producer. With the existence of a mobile gas phase, the pressure drawdown established at the production well is more efficiently propagated throughout the coalbed. Gas more efficiently propagates a pressure wave compared to water because gas is significantly more compressible. As the pressure decline within the coalbed continues, gas desorption, and therefore gas production, accelerates.
There is an important relationship between these two present production problems. The rate of gas diffusion from the coal can only be maximized by maintaining the lowest possible production well pressure, however, excessively low pressures increase water production. Conventional production practices overcome the diffusion-limited desorption of methane from the coal matrix by using such excessively low production well pressures, or do not set coalbed gas removal rates as disclosed in U.S. Pat. No. 4,544,037, allowing rate-controlling diffusion of coalbed gas and water encroachment to limit the economic life of the coalbed methane production well.
A related problem is coalbed structure water permeability. Increased water permeability allows water that is displaced from a coalbed to return more rapidly which results in increased waterhandling or a shorter economic lifespan of the coalbed reservior. Conventional production techniques do not effectively deal with the water permeability of the coalbed structure.
Another conventional coalbed gas production problem is the contamination of the coalbed gas removed from the coalbed with stimulation gas. As but one example, Amoco Production Co. (Amoco) has developed a method of increasing coalbed methane production by increasing the pressure difference between the coal matrix and the cleat system (diffusional, partial-pressure driving force) (U.S. Pat. No. 4,883,122). As that patent discloses, Amoco injects an inert stimulation gas (such as nitrogen) into an injection well. Nitrogen is less sorptive than coalbed methane and tends to remain in the cleat space. The injected nitrogen drives the resulting gas mixture to one or more producing wells, where the mixture is recovered at the surface. By the end of a year's production, the product gas may contain approximately 20 volume percent nitrogen. The simulated production rate profiles resulting from a continuous nitrogen injection are shown in FIG. 5. The point labeled P in FIG. 5 is the production rate immediately prior to application of the stimulation gas enhanced method. As is evident, the increase in gas production due to nitrogen injection is immediate and substantial. Much of the dramatic increase in early-time gas production results from the reduction in partial pressure of methane in the cleat system. Part of the improved recovery results from the increase in reservoir pressure that results from the injection of nitrogen into the coalbed. However, much of the production over the long term contains quantities of nitrogen which are substantially higher than minimum standards for pipeline natural gas.
Similarly, other ECBM methods which are designed to desorb gas by the injection of gas into an injection well and recover gas mixtures at one or more producing wells have high levels of contaminating stimulation gas in the coalbed gas removed at the production well. These techniques generally employ the use of CO.sub.2 or CO.sub.2 -nitrogen mixtures as disclosed by U.S. Pat. Nos. 5,454,666 and 4,043,395; and as disclosed in an Alberta Research Council (press release). CO.sub.2 is more sorptive than methane and tends to be adsorbed by the coal matrix. Therefore, the response of methane at the producers is attenuated. However, as with the above mentioned methods, these ECBM methods produce coalbed gas with high levels of stimulation gas. Therefore, as with the other above mentioned methods a gas cleanup process is required.
Another problem with injection of stimulation gas into a separate well located a distance from the production well is the production of increased water. In fact, Amoco's ECBM technique may increase overall water production because the increased quantity of coalbed gas that results from this injection-desorption process may tend to sweep additional quantities of water to the producer.
Yet another problem with convention coalbed gas production is high cost. Many of the above mentioned methods use stimulation gas at high pressure which requires the use of expensive, high-capacity, multistage gas compressors. Similarly, other methods also use high pressure as disclosed by U.S. Pat. Nos. 5,419,396; 5,417,286; and 5,494,108. High costs are also associated with the use of carbon dioxide gas as disclosed by U.S. Pat. No. 4,043,395, and in the continuous use of coalbed gases during coalbed gas production as disclosed by U.S. Pat. Nos. 4,883,122; 5,014,785; and 4,043,395.
Each of these problems of conventional coalbed gas production are addressed by the instant invention disclosed.